Transistors are used in numerous electronic devices in the world today. For example, transistors are often used in non-volatile memory (NVM) cells, which store data in a more permanent manner compared to volatile memory cells (such as random access memory). A conventional non-volatile memory cell often includes multiple transistors with a floating gate. A charge stored on the floating gate typically represents the logical value (a “1” or a “0”) stored in the non-volatile memory cell.
In a conventional non-volatile memory cell, the floating gate is often covered with an oxide film (such as tetraethylorthosilicate or “TEOS”) and a nitrided silicon oxide film (such as silicon oxynitride or “SiON”). An oxide (such as silicon dioxide) also often separates the floating gate from an underlying semiconductor substrate. In this type of non-volatile memory cell, the oxides ideally prevent electrons stored on the floating gate from being discharged unintentionally, which leads to failure of the non-volatile memory cell. However, it often remains possible for electrons stored on the floating gate to discharge to the semiconductor substrate through the oxide film and the nitrided silicon oxide film.
One approach to solving this problem involves selectively etching the nitrided silicon oxide film over the floating gate in a non-volatile memory cell, which makes the nitrided silicon oxide film thinner. This can reduce the discharge of electrons from the floating gate through the nitrided silicon oxide film, which may reduce the likelihood of failure of the non-volatile memory cell.
This approach, however, has various disadvantages. For example, etching silicon oxynitride films often requires the use of special tools, such as a poly etcher. Standard complimentary metal oxide semiconductor (CMOS) fabrication processes do not allow semiconductor wafers to be sent to a poly etcher after a silicon oxynitride film has been deposited (due to cobalt contamination or other issues). As a result, this approach may require a dedicated poly etcher that is used only to etch silicon oxynitride films in non-volatile memory cells. This would limit the use of the poly etcher, which is often an extremely expensive piece of equipment. While it is possible to use the same poly etcher to etch silicon oxynitride films in non-volatile memory cells and to process other CMOS devices, the poly etcher would need to be cleaned more frequently. For these reasons, this approach often increases the cost and time required to manufacture the non-volatile memory cells and other CMOS devices, decreases manufacturing efficiency, and increases material and labor costs.